Process for hardening articles made of iron or steel



Patented May 12, 1931 UNITED STATES PATENTA OFFICE BECK, OF FRANKFOR-T-ON-THE- MAIN, GERMANY, ASSIGNOR TO DEUTSOHE GOLD & SILBER SGHEIDEAZN'STALT, VORMALS ROESSLER, OF FRANKFORT-ON-THE- 1mm; GERMANY; A coaroRArIoN PROCESS; FOR HARDENING ARTICLES MADE OF IRON OB STEEL No Drawing. Application filed September 1, 1927, Serial No. 217,010, and in Germany September 6, 1826.

The present invention relates to'an improvement of the process of carburizing articles.

- made of iron or steel by means of fused hydrosurface of the iron article depends chiefly on the temperature at which the immersion takes place. I have also found that at lower temperatures the introduction ofthe nitrogen prevails whilst at higher temperatures the iron nitride formed is decomposed and the introduction of the carbon predominates. With cyanide mixtures of the type described above it was, however, not possible to exceed a certain temperature as above this temperastarts frothing o 161'.

ture the fused mixture is decomposed at too great a rate, so that in consequence the bath It loses its contents of carbon too rapidly. The carbon therefore cannot be utilizedand is lost.

I have found now that by admixing finely divided carbonaceous material such as wood charcoal, animal charcoal and the like to the carburizing mixture containing cyanide salts I was able to raise the temperature of the bath far in excess of the one towhich the cyanide mixture could be heated without such admixtures. Thus a mixture of sodium cyanide, soda carbonate and sodium chloride containing about 40% of sodium cyanide which has been'used largely for case harden- 7 ing at 850 C. commences frothing and rising over the top of the crucible when heated above 860 C. whilst a melt to which finely divided charcoal had been added in the proportion of for instance 3% to of the salt mixture could be heated to temperatures above 950 C.

without impunity. A temperature even of 1000? C. and more was attainable without the decomposition becoming so great that the melt was frothing over. When using temperatures of 960 to 970 C. with a bath containing sodium cyanide, soda carbonate and sodium chloride I have found for instance that the carbon penetrated to a depth approximately three times as great as when working without addition of finely divided carbon at about 850 C.

Instead of using finely divided materials containing carbon such as for instance wood charcoal and the like I can use substances which at the temperatures utilized decompose or have been decomposed with liberation of carbon. Substances of this kind are for in-. stance organic compounds of high molecular weight such as for instance higher hydrocarbons etc.

When the bath containing alkali metal cyanides such as sodium cyanides and finely divided carbonaceous material in accordance to my invention in admixture with other diluting agents has been in use for some time its contents of carbonates are increased gradually by the decomposition of the cyanides whilst its contents of efiicaceous cyanide decreases. In order to prevent the bath from becoming too poor in cyanide I have generated cyanides in the bath itself during the heating. For this purpose I add to the bath as diluting agent from the beginning in addition to finely divided carbonaceous material cyanamide of alkali metals or alkaline earth metals such as for instance sodium cyanamide. Under the influence of the temperatures used during the carburization the cyanamides with a corresponding part ofthe finely divided carbon present were converted into cyanides. I may utilize the cyanamide either in the place of or in addition .to other admixtures such as soda carbonate,

common salt or the like. This addition has the further advantage that the fusedbath is more fluid at the working temperature than a melt without it. As a certain amountof the fused cyanide mixture will adhere to the articles on removal from the bath, the amount removed from the melt will be less'the more liquid the mass will be. For this reason the bath according to my invention will last longmany this eighteenth 1927.

er than a bath of the kind hitherto known.

The carburized articles after removal from the bath are quenched in the well known way using water, brine or oil as quenching liquid.

Comparatively small quantities of the finely divided charcoal distributed in the fused sodium cyanide mixture are suflicient to obtain the effect aimed at and to prevent the melt from frothing over at temperatures above 920 C. Thus for instance I obtained excellent results with a mixture containing 85% sodium cyanide, 11 to 12% sodium cyanamide and 2.5 to 3% finely ground. wood charcoal, working at a temperature of 950 Example-Manufactured articles of steel containing 0.08% carbon were immersed in a fused mixture consisting of 85 parts sodium cyanide 12.0 parts sodium cyanamide and 3 parts finely ground wood charcoal and kept in the melt or 30 minutes. The carburized articles were then removed from the melt, allowed to drip and quickly quenchedin brine. The depth of penetration after that time is 0.4 millimetres ,whilst the surface is as hard as glass.

What I claim is:

1. A hardening mixture for hardening iron and steel containing sodium 0 anide, sodium cyanamide and about 3% ot finely divided charcoal distributedtherethrough.

2. Process for carburizing iron articles which consists in immersing said articles in a fused salt bath comprising an alkali metal cyanide and finely divided charcoal distributed therethrough, and holding said bath. and immersed articles at temperatures exceeding about 920 C, until carburization' has been efiected.

. 3. Process for carburizing iron articles which consists in immersing said articles in a fused salt bath comprising an alkali metal cyanide, a cyanamide of the group comprising alkali'metal and alkaline earth metal cyanamides, and finely divided carbonaceous material distributed therethrough, and holding said bath and immersed articles at temperatures between 920 and 1000 C. until ,carburization has been efiected.

Signed at Frankfort-on the-Main,

Prussia and State of Gerday of August, D.

WALTER BECK.

Hesse-Nassau, 

